Inkjet recording methods are advantageous in that, by virtue of a simpler process than that of other recording methods, the formation of full-color images is easy and, even in apparatuses having a simple construction, high-resolution images can be obtained.
Dye-based inks obtained by dissolving various water soluble dyes in water or a mixed liquid composed of water and an organic solvent are used as ink for inkjet. Dye-based inks can yield images having a sharp color tone but on the other hand, are poor in lightfastness.
On the other hand, pigment-based inks with carbon black or various organic pigments dispersed therein are superior in lightfastness to dye-based inks and thus are actively studied.
Pigment-based inks, however, are more likely to cause nozzle clogging than dye-based inks.
The pigment-based inks are generally prepared by preliminarily dispersing a coloring material and a dispersant in an aqueous solvent such as water or an alcohol to prepare a dispersed material, then subjecting the dispersion to a step of dispersing the dispersed material with a media-type disperser such as a sand mill to a predetermined degree, and then diluting the dispersion to give a predetermined concentration.
In pigment-based aqueous inks, in order to disperse hydrophobic pigments, it is common practice to use surfactants or water-soluble resins. However, the reliability of the formed images is very poor. Accordingly, a technique has been disclosed in which fine resin particles having a film forming property are added to an ink liquid with a view to improving the image quality. However, it is difficult to keep a plurality of ingredients in a finely and stably dispersed state for a long period of time. The use of a large amount of a dispersant such as a surfactant to stably disperse the fine particles disadvantageously poses problems such as the occurrence of air bubbles within ink tanks or heads or deteriorated image quality. Further, for dispersibility improvement purposes, studies have been made to adopt a method for converting groups on the surface of the pigment to hydrophilic groups or to use hydrophilic group-containing resins. In this technique, when a single type of pigment is used, the pigment can be stably dispersed, but on the other hand, when a plurality of types of pigments are mixed together, the dispersion is unstable, posing a problem of deteriorated storage stability.
Reports have been given on surface-treated pigment inks that can provide excellent print quality and also possess excellent ejection stability and storage stability (PTL 1, PTL 2, and PTL 3), ink sets that can maintain stable ink ejection (PTL 4), a method in which a water-insoluble coloring material and electrifiable pseudo-fine particles of resin having a size smaller than the coloring material are incorporated in inks with a view to obtaining a high image density (PTL 5), a method in which a self-dispersible pigment having a limited DBP oil absorption is incorporated (PTL 6), and an aqueous carbon black dispersion that comprises a surface-modified carbon black, has an HLB value of 7 to 18, and contains a nonionic surfactant having an acetylene skeleton (PTL 7). Further, aqueous inks using an acetylene glycol surfactant are also reported (PTL 8).
Further, disclosures have been made on a method in which a water-dispersible resin having carboxyl and nonionic hydrophilic group in its molecule is dispersed in water to stabilize dispersion liquid (PTL 9), a method in which a water-soluble polymer and a surfactant, which have the same polarity or one of which is nonionic, are added (PTL 10), a method in which, in an aqueous recording liquid, a colorable ionic group-containing polyester resin and a colorant, which are identical to each other in polarity of a hydrophilic group, are used (PTL 11), and a method in which a pigment and fine resin particles, which are identical to each other in polarity of dispersion, are used (PTL 12). Further, a printing ink using a gemini surfactant as a dispersant has been disclosed (PTL 13).
Further, a proposal has been made on an aqueous inkjet ink composition containing a pigment dispersion containing pigment particles having such a particle size distribution that at least 70% of the particles in the dispersion is accounted for by particles having a diameter of less than 0.1 μm and the other particles in the dispersion have a diameter equal to or less than 0.1 μm, an aldehyde naphthalene sulfonate dispersant, and/or at least one sulfone solvent (PTL 14). A proposal has also been made on a recording liquid containing a pigment, a polymeric dispersant, and a nonionic surfactant (PTL15). A proposal has also been made on the use of an AB or BAB block copolymer as a dispersant for pigments (PTL16 and PTL17). Further, a proposal has also been made on the use of a specific pigment, a water-soluble resin, and a solvent (PTL18).
On the other hand, proposals have been made on methods for dispersing pigments without use of a dispersant, that is, a method in which a water solubilizing group-containing substituent is introduced into carbon black (PTL 19), a method in which a water-soluble monomer or the like is polymerized on the surface of carbon black (PTL 20), and a method in which carbon black is oxidized (PTL 21). Further, a proposal has been made on a method in which waterfastness and ejection stability are ensured by an ink containing an oxidized carbon black and a terpolymer of acrylic acid, styrene, and α-methyl styrene (PTL 22).
Further, a proposal has also been made on an inkjet recording liquid containing dispersed particles having a volume average particle diameter of 30 nm to 200 nm (PTL 23).
Regarding the proposed conventional ink liquids, the color pigment inks can realize a high image density. However, the black pigment inks are unsatisfactory yet.
Further, a proposal has also been made on the use of beads having a diameter of 0.05 mm to 1.0 mm in beads mill dispersion liquid (PTL24 to PTL26). None of these proposals are satisfactory in dispersion stability.
In PTL27, an anionic surfactant is used as the dispersant, and the molecular weight is preferably in a range of 1,000≦m≦30,000. These do not provide satisfactory dispersion stability, and pigments species that are weak against impact exposed during dispersion lack in stability after the dispersion, posing a problem of, for example, ejection stability of ink liquids.